Polysilylenesiloxane polymers useful in enhanced oil recovery using carbon dioxide flooding

ABSTRACT

A polymer is disclosed that is useful in increasing the viscosity of carbon dioxide. That polymer comprises a polysilylenesiloxane polymer, preferably polydimethylsilylenedimethylsiloxane. That polymer can be used in a method for recovering oil from an underground oil-bearing earth formation penetrated by an injection well and a producing well, in which method carbon dioxide is injected into the formation to displace oil towards the producing well from which oil is produced to the surface. The viscosity of the carbon dioxide injected into the formation is increased at least three-fold by the presence of a sufficient amount of the polysilylenesiloxane polymer and a sufficient amount of a cosolvent to form a solution of the polymer in the carbon dioxide.

The present invention relates to a polysilylenesiloxane polymer usefulin increasing the viscosity of carbon dioxide; to new compositions ofmatter comprising carbon dioxide and a viscosifying amount of a mixtureof a cosolvent and that polysilylenesiloxane; and to a method ofrecovering oil from underground subterranean formations using those newcompositions of matter.

BACKGROUND OF THE INVENTION

In newly discovered oil fields, oil usually will be recovered by the oilflowing from a producing well under the naturally occurring pressure ofthe fluids present in the porous reservoir rocks. That naturallyoccurring pressure decreases as the fluids are removed. This phase ofproduction, called primary production, recovers perhaps 5% to 20% of theoil present in the formation.

Secondary recovery methods (e.g., waterflooding) are used to recovermore of the oil. In these methods, a fluid is injected into thereservoir to drive additional oil out of the rocks. Waterflooding haslimitations. Since the water is immiscible with oil, as the waterdisplaces the oil remaining in the reservoir reaches a limiting valueknown as "the residual oil saturation" and the oil stops flowing. Thereis a strong capillary action which tends to hold the oil in theinterstices of the rocks. The amount of oil recovered by secondarytechniques is usually from about 5% to 30% of the oil initially present.

In recent years, more attention has been directed to enhanced oilrecovery or tertiary recovery methods. These tertiary recovery methodsare used to recover the residual oil by overcoming the capillary forceswhich trap the oil during waterflooding. For example, it has beensuggested to add surfactants to the flood to decrease the interfacialtension and thus allow oil droplets to move to producing wells.

Secondary or tertiary recovery of oil is also possible by the misciblefluid displacement process. A number of carbon dioxide floods have beentried in the United States. The carbon dioxide tends to dissolve in theoil, which swells with a consequent decrease in viscosity andimprovement in the flow to producing wells. The carbon dioxide alsoextracts light hydrocarbons from the oil and this mixture of carbondioxide and light hydrocarbons can, in some cases, reach a compositionthat will miscibly displace the oil.

This carbon dioxide-rich phase characteristically has a lower viscositythan the oil and tends to finger through the formation. Early carbondioxide breakthrough is undesirable since reservoir sweep is reduced andexpensive separation procedures are required to separate and recycle thecarbon dioxide.

In U.S. Pat. No. 4,913,235, Harris et al. report a means of increasingviscosity for the carbon dioxide thirty-fold or more by usingcosolvents, along with certain defined polymers having a solubilityparameter of close to 6.85 (cal/cc)^(1/2) [14.0 J^(1/2) /cm^(3/2) ] andhaving electron donor groups such as ether, silyl ether, and tertiaryamine. Those defined polymers include polysiloxanes and polyvinylethers.

SUMMARY OF THE INVENTION

The present invention is based on the discovery thatpolysilylenesiloxane is especially useful in increasing the viscosity ofcarbon dioxide because of that polymer's high solubility in carbondioxide. Preferably, the polysilylenesiloxane ispolydimethylsilylenedimethylsiloxane orpolymethylethylsilylenedimethylsiloxane.

In one embodiment of the present invention, the polysilylenesiloxane isused in a method for recovering oil from an underground oil-bearingearth formation penetrated by an injection well and a producing well. Inthat method, carbon dioxide is injected into the formation to displacethe oil towards the producing well. The viscosity of that carbon dioxideis increased at least three-fold by the presence of a sufficient amountof the polysilylenesiloxane polymer and a sufficient amount of acosolvent to form a solution of the polymer in the carbon dioxide. It ispossible that, for some polysilylenesiloxane polymers, under someconditions of pressure and temperature, no cosolvent will be needed.

Preferably, the carbon dioxide solution comprises from 70 to 99.9 weightpercent carbon dioxide; from 0.05 to 20 weight percent polymer; and from0.05 to 30 weight percent cosolvent. The carbon dioxide solution can beprepared by forming a first solution of the polymer and the cosolventand then mixing carbon dioxide with the first solution.

In another embodiment, the polymer is a copolymer of silylenesiloxaneand dialkylsiloxane or silalkylene silane, preferably a copolymer ofdimethylsiloxane and dimethylsilylenedimethylsiloxane, or a copolymer ofdimethylsilmethylene dimethylsilane anddimethylsilylenedimethylsiloxane.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In its broadest aspect, the present invention is based upon thediscovery of a polymer that should have a better solubility in carbondioxide. That polymer is a polysilylenesiloxane polymer.

In one preferred embodiment of this invention, this polymer is used in amethod for recovering oil from an underground oil-bearing earthformation penetrated by an injection well and a producing well. In thatmethod, carbon dioxide is injected into the formation to displace theoil towards the producing well. The viscosity of the carbon dioxide thatis injected into the formation is increased at least three-fold by thepresence of a sufficient amount of a polysilylenesiloxane polymer and asufficient amount of a cosolvent to form a solution of the polymer inthe carbon dioxide. It is possible that, for some polysilylenesiloxanepolymers, under some conditions of pressure and temperature, nocosolvent will be needed.

POLYMER

The polymer of the present invention is a polysilylenesiloxane polymer.Since this polymer has a lower solubility parameter than thepolysiloxanes and polyvinylethers of Harris et al., it should be moresoluble in carbon dioxide.

By "polysilylenesiloxane," we mean a polymer having a repeating unitthat has in its backbone two adjacent silicon atoms, with each siliconatom having two pendant alkyl groups, and having in its backbone anoxygen atom which is singly bonded to the second of the two siliconatoms and to the first silicon atom of the next repeating unit in thebackbone. The four pendant alkyl groups can be either the same ordifferent.

The chemical formula of the repeating unit of polysilylenesiloxane is:##STR1## where R₁, R₂, R₃, and R₄ are alkyl groups. Preferably, R₁, R₂,R₃, and R₄ are either methyl or ethyl, more preferably, methyl.

The structures of the repeating units of the preferred polymers areshown in the table below:

    ______________________________________                                        TABLE OF PREFERRED POLYMERS                                                                                 Estimated                                                                     Solubility                                                                    Parameter                                       Repeating Unit                                                                             Polymer          (J.sup.1/2 /cm.sup.3/2)                         ______________________________________                                         ##STR2##    polydimethylsilylene- dimethylsiloxane                                                         14.2                                             ##STR3##    polymethylethylsilylene- dimethylsiloxane                                                      14.5                                             ##STR4##    polydiethylsilylene- dimethylsiloxane                                                          14.7                                             ##STR5##    polymethylethylsilylene- methylethylsiloxane                                                   14.7                                            ______________________________________                                    

Most preferably, the polymer is eitherpolydimethylsilylenedimethylsiloxane orpolymethylethylsilylenedimethylsiloxane. Those two polymers haveestimated solubility parameters of 14.2 and 14.5, respectively, ascompared to polydimethylsiloxane, which has an estimated solubilityparameter of 15.3.

The preparation of polysilylenesiloxanes is well known in the art. Forinstance, it is taught in W. NOLL, CHEMISTRY & TECHNOLOGY OF SILICONES,Academic Press, Orlando, 1968, pg. 368; and H. A. Clark's German Patents(917,275 and 917,276).

In another embodiment, the polymer can be a copolymer having asilylenesiloxane repeating unit and either a siloxane repeating unit ora silalkylenesilane repeating unit. By "copolymer" we mean a polymerhaving more than one type of repeating unit. The repeating units of thecopolymer can be incorporated either randomly, regularly, in blocks, asgrafts, or combinations thereof. By "randomly" we mean that thesilylenesiloxane and siloxane or silalkylenesilane repeating units areattached to the backbone of the polymer in a disordered sequence (e.g.,AABABBBAB, etc.). By "regularly" we mean that the silylenesiloxane andsiloxane or silalkylenesilane repeating units are attached to thebackbone alternately (e.g., ABABABAB, etc.). By "in blocks" we mean thatthe silylenesiloxane and siloxane or silalkylenesilane repeating unitsare attached in segments (e.g., AAABBBBBAA, etc.). By "graft" we meanthat the silylenesiloxane or siloxane or silalkylenesilane units areadded as side chains ##STR6##

The silylenesiloxane repeating units are more fully described above.Examples of suitable siloxane repeating units are shown in Harris etal., cited above, which is hereby incorporated by reference for allpurposes. Preferably, the siloxane repeating unit is dimethylsiloxane.Preferably the silalkylenesilane repeating unit is dimethylsilmethylenedimethylsilane. The preparation of useful copolymers is disclosed in W.NOLL, CHEMISTRY & TECHNOLOGY OF SILICONES, Academic Press, Orlando,1968, pg. 367.

CARBON DIOXIDE

The carbon dioxide can come from any suitable source, such as thosedescribed in "Miscible Displacement" by Fred I. Stalkup, Jr. (MonographVol. 8, Henry L. Doherty Series, ISBN NO895203197, Society of PetroleumEngineers, 1983, Chap. 8, sec. 8.4). Substantially pure carbon dioxideis preferred, but water-saturated carbon dioxide is acceptable sincewater (or brine) is usually present in the formation. Usually, thecarbon dioxide contains at least 95% carbon dioxide and preferably atleast 98% carbon dioxide, the remainder being usually lighthydrocarbons. The amount of impurities in the carbon dioxide which canbe tolerated is a function of the type of oil to be displaced and thetype of displacement operation.

COSOLVENT

As a class, it would appear that many materials are suitable for use ascosolvents in this invention:

(a) alcohols having from 1 to 8 carbon atoms,

(b) aromatics having a single ring and from 6 to 10 carbon atoms,

(c) ketones having from 3 to 10 carbon atoms,

(d) carboxylic acid esters where the carboxylic acid portion has from 2to 4 carbon atoms and the ester portion has from 1 to 10 carbon atoms,

(e) hydrocarbons having from 2 to 20 carbon atoms, including refinerystreams such as naphthas, kerosene, gas oils, gasolines, etc., and

(f) ethers and glycol ethers having from 2 to 10 carbon atoms.

If hydrocarbons are used, preferably they are aliphatic, naphthenic, oraromatic hydrocarbons having from 3 to 10 carbon atoms.

One technique for obtaining the desired cosolvent on-site is to contactthe carbon dioxide in a liquid-liquid extraction apparatus withrecovered crude or a fraction of such crude for a sufficient time topermit the carbon dioxide to extract enough light hydrocarbons tofunction as the cosolvent. The desired amount of polymer would then beadded to the carbon dioxide-light hydrocarbon extract to form theoil-driving material.

The polymer-cosolvent-carbon dioxide mixture must be compatible with theformation fluids so that the polymer won't precipitate in the formationafter injection.

AMOUNTS OF CARBON DIOXIDE, POLYMER, AND COSOLVENT

The new compositions of this invention preferably comprise from 70 to99.9 weight percent carbon dioxide and a sufficient amount of a mixtureof a polymer and a cosolvent to effect at least a three-fold increase inthe viscosity of the carbon dioxide. Usually the weight percent polymerin the mixture is from 0.05 to 20 weight percent. The amount ofcosolvent is at least sufficient to dissolve the desired amount ofpolymer in the carbon dioxide and is at least 40% by weight of thepolymer employed. This amount of cosolvent is usually from 0.05 to 30weight percent of the final mixture.

One method of preparing the viscous carbon dioxide solution is byforming a first solution of the polymer and cosolvent and then mixingcarbon dioxide with the polymer-cosolvent solution. The viscous carbondioxide is displaced through the formation by a drive fluid which iscomprised of slugs of viscous carbon dioxide alternated with slugs of afluid comprising water or reservoir brine.

While the present invention has been described with reference tospecific embodiments, this application is intended to cover thosevarious changes and substitutions which may be made by those skilled inthe art without departing from the spirit and scope of the appendedclaims.

What is claimed is:
 1. In a method for recovering oil from anunderground oil-bearing earth formation penetrated by an injection welland a producing well, in which method carbon dioxide is injected intosaid formation to displace oil towards said producing well from whichthe oil is produced to the surface, the improvement comprising injectinginto said formation carbon dioxide, the viscosity of which is increasedby the presence of a sufficient amount of a polysilylenesiloxane polymerto increase the viscosity at least three-fold.
 2. The method accordingto claim 1 wherein said polymer is polydimethylsilylenedimethylsiloxane.3. The method according to claim 1 wherein said polymer ispolymethylethylsilylenedimethylsiloxane.
 4. The method according toclaim 1 wherein a sufficient amount of a cosolvent is present in saidcarbon dioxide to form a solution of said polymer in said carbondioxide.
 5. The method according to claim 4 wherein the carbon dioxidesolution comprises from 70 to 99.9 weight percent carbon dioxide; from0.05 to 20 weight percent polymer; and from 0.05 to 30 weight percentcosolvent.
 6. The method according to claim 4 in which said carbondioxide solution is prepared by forming a first solution of said polymerand said cosolvent and then mixing carbon dioxide with said firstsolution.
 7. In a method for recovering oil from an undergroundoil-bearing earth formation penetrated by an injection well and aproducing well, in which method carbon dioxide is injected into saidformation to displace oil towards said producing well from which the oilis produced to the surface, the improvement comprising injecting intosaid formation carbon dioxide, the viscosity of which is increased bythe presence of a sufficient amount of a polymer to increase theviscosity at least three-fold; wherein said polymer comprises acopolymer of a silylenesiloxane and a dialkylsiloxane.
 8. In a methodfor recovering oil from an underground oil-bearing earth formationpenetrated by an injection well and a producing well, in which methodcarbon dioxide is injected into said formation to displace oil towardssaid producing well from which the oil is produced to the surface, theimprovement comprising injecting into said formation carbon dioxide, theviscosity of which is increased by the presence of a sufficient amountof a polymer to increase the viscosity at least three-fold; wherein saidpolymer comprises a copolymer of a silylenesiloxane and asilalkylenesilane.